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February 1, 2002

6 Min Read
The taming of the screw

When Dave Larson, president of Westland Corp., which designs custom screws to meet specific job requirements, pictures the proper mixing of material by a screw, he always sees the same image. 

"Basically, we're trying to knead the material," Larson explains, "just like our grandmothers would do with bread. You want to be very gentle with it. If you take bread and you put it in a mixer with beaters, the bread will never rise." 

As president of a company wholly focused on screw and barrel technology, it's Larson's job to assess and study the fundamental role screw design plays in high-quality molding. Unfortunately, he says many molders pay little, if any, attention to the topic. 

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The Eagle mixing screw, right, solved the streaking problems in these automotive parts. Made from acetal and colored using concentrates, the top part displays streaking, adjacent to the arrow, while the lower part shows the streakless results after the switch to the new screw.


"Screw and barrel design with a lot of molders is not a high priority," Larson says, "or it's not thought that a screw can make the kind of difference that it truly can make." 

There was a time when John Rake, vp and gm of Avon Plastic Products, would have fallen into this category of molders largely indifferent to the importance of the screw. Avon, a custom molder serving the automotive interior and trim market, uses a fair amount of color concentrates in its products. 

Homogeneous mixing is a necessity, and Rake had successfully color concentrated ABS and PP without considering the screw design. If he had irregularities, Rake would tweak backpressures and use dispersion disks and mixing nozzles to achieve a thorough blend. These adjustments worked fine with ABS and PP, but acetal was another story. 

"I'd been trying for years to do [color concentrating] with acetal without any success," Rake says. 

At one point, Rake tried cranking up backpressure to 350 psi, which created a uniform mix of acetal and colorant but placed the machine and the material under too much stress. His options were limited. Rake could run his machines at pressures he admitted could "blow the endcap off," or switch to expensive precolored compounds instead of using color concentrates. Yet there was another solution—one introduced by Westland that involved closer consideration of the screw being used in the process. 

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Avon uses at-the-throat mixing to introduce color concentrates to its resins.

A Whole New World 
To help Rake—who had been running all of his machines with the standard screws that came with the machines—realize the benefits of a more specialized screw, Westland sent out a lengthy project and process profile form for the molder to fill out. 

"[The profile] was quite extensive in terms of what we were trying to do, what the melt flows were, what we were running at, and what we considered optimal in terms of backpressures, melt temperatures, and injection pressures," Rake says. "We forwarded this four- to six-page questionnaire back to them, and they came back with a proposal for an Eagle mixing screw." 

After specifying the screw, Westland sent consultant Terry Williams to Avon for installation and training. The new mixing screw quickly proved to be the right choice. 

"We're now successfully color-concentrating acetal," Rake says. Not only that, but the Eagle screw is also specifically designed for the process profiles Rake required. The Eagle's mixing section uses wiper flights with large helix angles to force the melt over barrier lands or through one of six mixing notches (see diagram below). This motion encourages thorough mixing without the excessive shear that can degrade material. 

"We didn't want [the Eagle] to be a restrictive device," Larson says. "We didn't want it to hold the material back on the screw; we wanted it to flow freely. So the mixer is designed to handle 100 percent of the material that is coming up the screw." 

Larson says Westland accomplished this by reducing the mixer's root diameter and by increasing the helix angle of the flights. Instead of the typical 17.5° square pitch design, the Eagle uses 40° and 50° helix angles on two different flights. The material begins at two inlet channels that start wide and then gradually narrow to compress the material. As the channel is narrowed and the material is compressed, it passes through a mixing notch and into a slender channel that widens. This creates the aforementioned kneading action. 

"We don't want to destroy the polymer structure," Larson says, "so we want to be very gentle with it, but knead it and mix it so we have good uniform product when it comes out the end." 

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The arrows show the flow of resin over the barrier lands and through the mixing notches, creating the "kneading" action.

Seeing the Light 
A true convert, Rake says he has gone from totally neglecting his screws to seeing them as an integral piece of the molding puzzle. 

"I really thought that in my 22 years in this business I'd gotten a handle on what made a good part," Rake says, "but when [Williams] came out and educated us on plasticating, I realized how little I knew about the whole area." 

Rake says his epiphany came in two primary areas. First, at the suggestion of Williams, Rake has installed gauges on all his machines to monitor pressure during screw recovery. This isn't standard operating procedure for many molders, but Rake says it offers another view of the process that helps monitor the aging machines, the newest of which dates back to 1985. 

"I've got older-technology molding machines," Rake says, "but if we see a blip now either up or down in our screw recovery, that's a more instantaneous indicator to us that something has gone wrong in the process—we've blown a heater band, or something else is awry." 

Light has also been shed on the effect compression ratios have on amorphous and crystalline materials. Previously, Avon exclusively used 3:1 compression ratio screws, and the disparate melt points of amorphous and crystalline materials reacted differently to the prolonged shearing action. Rake realized he was neglecting the needs of amorphous materials, so he compromised and switched to a 2.6:1 screw. 

"Our compression ratio has shifted more towards being friendly with amorphous materials, and yet our crystallines can still handle it," Rake says. 

So changed is Rake's mentality that if he perceives a potential benefit, he'll trade out screws as he changes molds, trying to match the right screw with the right job. 

"I'm not losing any downtime on the molding machine," Rake says, "and by switching to these high-performance screws specifically designed for certain resin systems, I'm picking up cycles; I'm picking up quality; I'm picking up dimensional stability. The benefits are just tremendous." 

The financial benefits have been impressive as well. By spending $.50/lb less on the 225,000 lb of acetal Avon processes annually, Rake now saves about $125,000 a year in material costs on acetal alone. 

"It's kept us in the ball game," Rake says. "I thought I had gotten all the low-hanging fruit out there, and that we were running a pretty lean manufacturing operation, but when Westland got us involved in taking a look at the whole plasticating process, we found money." 

Contact information
Westland Corp.
Wichita, KS
Dave Larson
(316) 721-1144
www.westlandusa.com

Avon Plastic Products Inc.
Rochester Hills, MI
John Rake
(248) 852-1000

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